Pressure-Induced Metallization and Electrical Phase Diagram for Polycrystalline CaB_6 under High Pressure and Low Temperature

The electrical properties of polycrystalline CaB_6 are revealed by in-situ resistance measurements under high pressure and low temperature. Due to the existence of grain boundaries, polycrystalline CaB_6 behaves with semiconducting transport properties, which is different from the semimetallic CaB_6 single crystals. The temperature-dependent resistance measurement results show that before the structural phase transition at 12.3 GPa the high pressure first induces the metallization at 6.5 GPa for CaB_6. Moreover, the phase diagram for CaB_6 is drawn based on the investigated electric conducting properties and at least three different conducting phases are found even at moderate high pressure and low temperature, indicating that the electric nature of CaB_6 is very sensitive to the environment.